1. Field of the Invention
The present invention relates to a spark ignition type internal combustion engine which determines (estimates) the self-ignition property of a fuel whose property is unknown.
2. Description of Related Art
Determining the self-ignition property (i.e. characteristic indicating the self-ignition property represented by octane number or cetane number) of a fuel using knocking determination (i.e. measurement of knocking limit ignition timing) is a well-known technology in the field of the spark ignition type internal combustion engine. Such determining has been widely used in discriminating high-octane gasoline from regular gasoline.
A related art spark ignition type internal combustion engine includes two kinds of fuels different in octane number that are blended and supplied to a combustion chamber. In the internal combustion engine, the octane number of a low octane fuel is measured, and then the two kinds of fuels are blended to measure the octane number of the blended fuel. The octane number of a high octane fuel is measured according to these octane numbers and a mixture ratio of the two kinds of the fuels.
According to the related art engine, each time when a fuel having an unknown octane number is supplied, the octane numbers of two kinds of fuels need to be determined. If the knocking is determined upon only the low octane fuel, there occurs such a problem that the knocking is intensified when a fuel having an extremely low octane number is supplied.